This invention relates to a process and an apparatus for cleaning nuclear reactor cooling water in atomic power plants, and particularly to a process and an apparatus for cleaning nuclear reactor cooling water in a filtration-desalting system using powdery ion exchange resin capable of reducing the radiation exposure of operators working in an atomic power plant and of being readily susceptible to waste disposal.
With recent increase in the number of atomic power plants in operation, reduction in radiation exposure of operators during the normal operating period and the periodic inspection period has been keenly desired, and thus it is necessary to efficiently remove radioactive materials contained in the nuclear reactor cooling water, such as fine particles having particle sizes of about 0.1-10 .mu.m, composed mainly of iron oxides called "cruds", or radioactive metal ions such as .sup.60 Co.sup.2+, .sup.59 Fe.sup.2+, etc.
In a boiling water-type nuclear reactor, the cooling water recovered by a condenser after the driving of a turbine has been so far cleaned by a filteration desalter precoated with powdery ion exchange resin and an ordinary desalter using a mixed bed of granular cation exchange resin and anion exchange resin, and fed to the nuclear reactor. The powdery ion exchange resin used in the filtration desalter has been a powdery mixture of pulverized cation exchange resin and anion exchange resin. Usually, the filtration desalter and the desalter together are generally referred to as "apparatus for cleaning nuclear reactor cooling water".
Reasons why the apparatus for cleaning nuclear reactor cooling water has a remarkable effect upon reduction in the radiation exposure will be described in detail below.
The main cause for radiation exposure is deposition of radioactive cruds and radioactive .sup.60 Co.sup.2+ on the piping, and the piping dosage is increased thereby. Furthermore, a cause for forming radioactive cruds and .sup.60 Co.sup.2+ is radioactivation of non-radioactive iron or cobalt dissolved from the condenser or piping into cooling water through neutron irradiation in the nuclear reactor. Thus, the reduction in the radiation exposure can be made by removing iron and cobalt in both crud and ion states in cooling water, irrespective of the radioactive or non-radioactive nature. Thus, apparatuses for cleaning nuclear reactor cooling water, based on a combination of a filtration desalter and a desalter, have been used, where the filtration desalter provided on the upstream side removes cruds and metal ions as radioactive materials in the cooling water at the same time, whereas the desalter removes the remaining metal ions which have not been completely removed in the filtration desalter.
In the foregoing prior art, the filtration desalter and the desalter use benzenesulfonic acid-based cation exchange resin and quaternary ammonium-based anion exchange resin as the powdery or granular ion exchange resins. Their molecular structures are shown below: ##STR1##
The reasons why the benzenesulfonic acid-based resin is selected as a cation exchange resin and the quaternary ammonium-based resin as an anion exchange resin among so many kinds of ion exchange resin are that they have a good heat resistance and a good radiation resistance, and moreover the benzenesulfonic acid-based resin is strongly acidic and the quaternary ammonium-based resin is strongly basic, so that they have a distinguished ability to remove neutral salts such as NaCl, etc., if present in the cooling water due to a leakage of sea water from the condenser.
As described above, use of an apparatus for cleaning cooling water, which comprises a filtration desalter and a desalter, can considerably reduce the radiation exposure of operators working in an atomic power plant. However, still much more reduction in the radiation exposure has been nowadays desired.
There has been proposed a process for using weakly acidic ion exchange resin, for example, weakly acidic cation exchange resin having carboxyl groups as ion-exchanging groups in an apparatus for cleaning nuclear reactor cooling water [Japanese Patent Application Kokai (Laid-open) No. 58-76146]. However, as a result of extensive studies made by the present inventors, it has been found that a portion of the weakly acidic cation exchange resins combined with non-radioactive metal ions leaks into the cooling water and deposits onto fuel rods, as in the case of benzenesulfonic acid-type cation exchange resin, and the non-radioactive metal ions are radioactivated, and redissolved into the cooling water to make deposition onto pipings and increase the radiation dosage as will be described in detail later.
To clasify the causes for the phenomena, the present inventors have made further studies and have found that, among the weakly acidic cation exchange resins, those whose ion-exchanging groups are directly bonded to the benzene rings have a relatively high bonding energy and show the similar phenomena to those of the sulfonic acid type resins. As a result of further studies, it has been found that only weakly acidic cation exchange resins whose ion-exchanging resins have a bonding energy of not more than 300 KJ/mole are effective for cleaning the nuclear reactor cooling water. The present invention is based on the said findings.